It's not as much his weight as his playing style. Yes, he can/should lose a few pounds, maybe 10. But 25lbs is way too much.
Most bigs the size of Cousins do not burst into passing lanes or explode and chase loose balls. Bigs his size don't usually ISO, dribble drive, cross their defenders up and attack the rim.
I think the best thing Boogie can do for himself is to develop an unstoppable go-to move from 15 feet inward. Turnaround fadaway from either side (many great bigs have used it). An iteration of the "Sky Hook", yes, this dude has the hands and touch to do it. Cut down on the try to shake someone guard moves that he loves and unfortunately, refrain from burst of speed moves. Instead, defend with superior positioning and spacing. There are other ways for guys that are slower and less athletic like Bogat and Gasol to be effective defensively.